Mike Lyle wrote:
As long as they smell sweet, and haven't turned acid and developed
green algae and fungal growth inside. I cheerfully reuse the previous
year's growbag stuff in this way, but I've never tried it with
two-year-old second-hand material.

I also often use the grow-bag compost for pots, although always making sure
never to pot tomatoes in ex-grow-bag material.

But last year all of my grow bags had mushrooms growing under the tomatoes.
:-( So the growbags this year got tipped onto the garden (which improve the
soil immensely, IME)